Book - A textbook of histology, including microscopic technic (1910) Special Histology 9
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Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.
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IX. The Organ Of Hearing
THE ear, the organ of hearing, consists of three parts : (i) The external ear, including the pinna or auricle and the external auditory canal ; (2) the middle ear, tympanum, or tympanic cavity, containing the small ear bones and separated from the external auditory canal by the tympanic membrane, but communicating with the pharynx by means of the Eustachian tube ; (3) the inner ear, or labyrinth, consisting of a bony and a membranous portion, the latter lined by epithelial cells, especially differentiated in certain regions to form a neuro-epithelium, in which the auditory nerves terminate. The first two parts serve for the collection and transmission of the sound-waves ; the complicated labyrinth, with its differentiated neuro-epithelium, for the perception of the same. Figure 366 presents in a schematic way the relationships of the parts here mentioned.
A. The External Ear
The cartilage of the ear, including that of the external auditory passage, is of the elastic variety, but differs from typical elastic cartilage in that it contains areas entirely free from elastic fibers. The elastic reticulum is, however, never absent near the perichondrium. The skin covering the pinna is thin, and in it are found hairs with relatively large sebaceous glands ; sweat-glands are found on the outer surface.
The skin lining the cartilaginous portion of the external auditory canal is somewhat mobile and possesses very few. pronounced papillae, and is characterized by the presence of so-called ceruminous glands, which represent modified and very highly differentiated sweat-glands. They are branched, tubulo-alveolar glands (Huber). They empty either into the hair follicles near the surface of the skin or on to the surface of the skin in the neighborhood of the hair follicles.
The skin lining the osseous portion of the external auditory canal is supplied with neither hair nor glands, and possesses slender papillae, especially in the neighborhood of the tympanic membrane. The corium is closely attached to the periosteum.
The tympanic membrane consists of a tense and a flaccid portion. It forms a part of both the external and the middle ear. From without inward, the following layers may be differentiated : (i) the cutaneous layer ; (2) the lamina propria ; and (3) the mucous layer.
The epidermis of the cutaneous layer is identical in structure with that of the outer skin, except that the superficial layers of the stratum corneum contain nucleated cells. The corium is very thin, except along the course of the manubrium of the malleus, where it is thickened, forming the so-called cuticular ridge, which possesses papillae and is supplied with vessels and nerves.
The lamina propria ends peripherally in a thickened ring of fibroelastic tissue, the annulus fibrosus, which unites at the sulcus tympanicus with the periosteum of the latter. The lamina propria is composed of connective-tissue fibers, in which two layers may be distinguished externally, the radiate fibers, the stratum radiatum, and internally, the circular fibers, the stratum circulars. The external radiate layer extends from the annulus to the umbo and manut>rium, and is interrupted in the flaccid portion of the tympanic membrane by the upper fourth of the manubrium and the short process of the malleus ; it gradually thins out toward the center until it finally disappears in the vicinity of the umbo. The fibers of the inner (circular) layer are circularly disposed. This layer is thickest at the periphery of the tympanic membrane, becoming gradually thinner toward the lower end of the manubrium, where it disappears. Between the two layers of the lamina propria is a small quantity of loose connective tissue. The manubrium of the malleus is inclosed within the tympanic membrane. This is due to the union of the fibers of the radial layer with the outer strata of the manubrial perichondrium, the handle of the malleus being here covered by a thin layer of cartilage. In the posterior upper quadrant of the tympanic membrane the two layers of the lamina propria intermingle, forming irregularly disposed bundles and trabeculae, the dendritic fibrous structures of Gruber.
Fig. 366. Schematic representation of the complete auditory apparatus (Schwalbe).
The mucous layer of the tympanic membrane consists of simple squamous epithelium separated from the lamina propria by a thin connective-tissue layer containing but few cells. It likewise extends over the handle of the malleus. In the flaccid portion of the tympanic membrane the lamina propria disappears, so that in this region the cutaneous layer and the mucous membrane are in direct contact.
B. The Middle Ear
The middle ear, or tympanum, is a small irregular cavity, filled with air, situated in the petrous portion of the temporal bone between the bony wall of the inner ear and the tympanic membrane, and communicates with the pharynx through the Eustachian tube. It contains the small bones of the ear, their ligamentous attachments, and, in part, the muscular apparatus moving them.
The mucous membrane lining the tympanic cavity is folded over the ossicles and ligaments of the tympanum and is joined to that of the tympanic membrane and the Eustachian tube, the line of junction with the former being marked by the presence of papilla-like elevations.
The epithelium of this mucous membrane is a simple pseudostratified ciliated epithelium, having two strata of nuclei. Cilia are, however, lacking on the surface of the auditory ossicles, on their ligaments, and on the promontory of the inner wall, as well as on the tympanic membrane. The mucosa of the mucous membrane is intimately connected with the periosteum, and may now and then contain short isolated alveolar glands, especially in the neighborhood of the opening of the Eustachian tube.
The "auditory ossicles" are true bones with Haversian canals and lamellae ; with the exception of the stapes, they contain no marrow-cavity. Very distinct perivascular spaces are seen surrounding the vessels in the canals (Rauber). The malleus articulates with the incus, both articular surfaces being covered with hyaline cartilage. Within this articulation we find a fibrocartilaginous meniscus, and at the summit of the short limb of the incus another small cartilage plate. Between the lenticular process of the incus and the capitulum of the stapes is another articulation, also provided with cartilaginous articular surfaces. The basal plate of the stapes is covered both below and at its edges with cartilage, as are also the margins of the fenestra ovalis (fenestra vestibuli). The basal plate is held in place within the fenestra by an articulation, provided with tense ligamentous structures on the tympanic and vestibular sides. Between these the connective tissue is quite loose. All the cartilaginous portions of the auditory ossicles, with the exception of the articular cartilages, rest on the periosteum (Riidinger, 70).
Thcfenestra rotunda (fenestra cochleae) is closed by the secondary or inner tympanic membrane, a connective-tissue membrane containing vessels and nerves, the outer wall of which is covered by ciliated epithelium, the inner (the surface toward the scala tympani) by flattened endothelial cells.
In the antrum and mastoid cells, the mucosa of the mucous membrane is immovably fixed to the periosteum. The epithelium is of the simple squamous variety and is nonciliated.
Fig. 367. Cross-section of the Eustachian tube with its surrounding parts ; X I2 (from a preparation by Professor Riidinger).
The mucous membrane of the osseous portion of the Eustachian tube is very thin, and its mucosa is intimately connected with the periosteum. Its epithelium is of the simple pseudostratified ciliated variety, having two strata of nuclei. There are rio glands. The mucous membrane of the cartilaginous portion of the Eustachian tube is thicker, and its epithelium, which is of the stratified ciliated variety, is higher, and often contains goblet-cells. Lymphoid tissue may be demonstrated in the mucosa of this portion, and occasionally structures resembling lymph-nodules are found, especially in the vicinity of the pharyngeal opening of the tube. In the cartilaginous portion of the. tube are mucous glands, which are particularly numerous in the vicinity of the pharyngeal opening (Riidinger, 72, 2). The cartilage of the Eustachian tube is in part yellow elastic^ in part hyaline, and in certain portions presents the appearance of white fibre-cartilage.
C. The Internal Ear
The internal ear consists of an osseous and a membranous portion, the osseous and the membranous labyrinths ; the latter is contained within the former, and, although smaller, presents the same general shape. The two structures are separated by a lymph-space containing the perilymph.
Fig. 368. Right bony labyrinth, viewed from outer side : The figure represents the appearance produced by removing the petrous portion of the temporal bone down to the denser layer immediately surrounding the labyrinth (from Quain, after Sommering).
In the bony labyrinth we recognize a central portion of ovoid shape, known as the vestibule, the outer wall of which forms the inner wall of the tympanum and presents two openings, the fenestra ovalis and the fenestra rotunda, separated by a ridge known as the promontory. This ridge becomes continuous with the lower portion of the bony cochlea, anterior and mesial to the vestibule and having the shape of a blunt cone. From the posterior portion of the vestibule arise three semicircular canals, known respectively as the external or horizontal semicircular canal, the anterior superior vertical, and the posterior inferior vertical semicircular canals. The canals communicate with the vestibule by means of five openings, the superior contiguous portions of the anterior and posterior canals uniting to form the canalis communis before reaching the vestibule. The three canals present near their origin from the vestibule enlargements known as the osseous ampullae. The osseous labyrinth is lined throughout by a thin -layer of periosteum, covered by a layer of endothelial cells.
The membranous labyrinth differs in shape from the osseous labyrinth in that, in place of the single chamber (vestibule) of the latter, the membranous labyrinth presents two sacs, the utriculus and the sacculus, united by a narrow duct, the utriculosaccular duct. The utriculus is the larger, and from it arise the membranous semicircular canals. These present ampullae, situated within the osseous ampullae previously mentioned. The sacculus communicates with the cochlear duct by means of the canalis reuniens (Hensen). From the utriculosaccular duct arises the ductus endolymphaticus, which passes through the aqueductus vestibuli and ends in a subdural sacciis endolymphaticus on the posterior surface of the petrous portion of the temporal bone.
In the membranous labyrinth the nerves are distributed over certain areas known as the maculce, cristce, and papilla spiralis.
Fig. 369. Membranous labyrinth of the right ear from five-month human embryo (from Schwalbe, after Retzius).
There is a macula within the recess of the utriculus, the macula acustica utriculi ; and another within the sacculus, the macula acustica sacculi ; cristae are present in the ampullae of the upper, posterior, and lateral semicircular canals, the cristce ampullares sup., post., et lat. Besides these, we have the terminal arborization of the acoustic nerve in the membranous cochlea, the papilla spiralis cochlece, or the organ of Corti.
1. Utriculus and Sacculus
Only the inner wall of the utriculus is connected with the periosteum of the vestibule. In this region lies the corresponding macula cribrosa, through which the nerves penetrate to the macula of the utriculus. The utriculus and sacculus fill only a part of the inner cavity of the osseous vestibule. Between the osseous and membranous portions remains a space traversed by anastomosing connective -tissue trabeculae, and lined by endothelium, which also forms an investing membrane around the trabeculae. These trabeculae pass on the one side into the periosteum lining the vestibule, and on the other, into the wall of the utriculus and sacculus. The cavity which they thus traverse represents a perilymphatic space. (Compare Fig. 370, which shows analogous relations in the semicircular canals.)
Fig. 370. Transverse section through an osseous and membranous semicircular canal of an adult human being; y<^5 (after a preparation by Dr. Scheibe): a, Connectivetissue strand representing a remnant of the embryonic gelatinous connective tissue. Such strands serve to connect the membranous canal with the osseous wall.
The wall of the utriculus, especially its inner portion, consists of dense fibrous connective tissue, most highly developed in the region of the macula acustica. In the immediate vicinity of the macula utriculi the epithelium of the utriculus is high columnar in type ; in the remaining portion it consists of a single layer of low columnar cells, with a distinct basement membrane ; the epithelium of the macula itself is also high, and is composed of two kinds of elements of sustentacular elements and of the so-called auditory hair-cells. The sustentacular cells are tall epithelial cells resting on the basement membrane by means of their single or cleft basal plates. Each possesses an oval nucleus lying at or beneath the center of the cell. The hair-cells are peculiar cylindric elements with somewhat thickened and rounded bases. One end extends to the surface of the epithelium, while the other, which contains the nucleus, extends only to the center of the epithelial layer. The free end is provided with a cuticular zone supporting a number of long, stiff hairs, which often coalesce to form single threads. On the surface of the epithelium, which must be regarded as a neuro-epithelium, are crystals of calcium carbonate, known as oto-* lit/is, each of which incloses a minute central vacuole (Schwalbe). The otoliths are inclosed in a homogeneous substance, the otolithic membrane, which coagulates in a network of filaments when subjected to the action of fixing agents.
The nerve-fibers going to the macula penetrate the wall, and, under the epithelium, undergo dichotomous division, and, after further division, form, in the region of the basilar ends of the auditory cells, a plexus consisting of fine ramifications, and embracing the lower ends of the auditory cells. A few fibers extend still further upward, where their telodendria enter into intimate relations with the acoustic cells (v. Lenhossek, 94, i).
The structure of the sacculus is in every respect like that of the utriculus, and a further description of it is therefore unnecessary.
2. The Semicircular Canals
The membranous semicircular canals are attached at their convex surfaces to the periosteum of the bony canals, which they only partly fill, the remaining cavity being occupied by an eccentrically situated perilymphatic space traversed by connective-tissue trabeculae. The walls of the perilymphatic spaces of the semicircular canals, like those surrounding the utriculus and the sacculus, are lined by endothelium, which covers, on the one hand, the periosteal surface of the bony semicircular canals, and, on the other hand, the outer wall of the membranous canals, together with the connective-tissue trabeculae. The connective-tissue walls of the membranous canals are structurally similar to those of the utriculus and sacculus. Hensen compares their structure to that of the substantia propria of the cornea. In the adult, the inner layer of the wall of the canals supports here and there papillary elevations, which, however, disappear along its attachment to the bony semicircular canal (Riidinger, 72, 88).
The epithelium lining the membranous semicircular canals is simple squamous in character and very evenly distributed over the entire inner surface, including the papillae previously mentioned.
On the concave side of each semicircular canal the epithelial cells are somewhat narrower and higher. This inner and higher epithelium (raphe), extending along the concave side into the ampullae, marks the region at which the semicircular canals were constricted off from the pocket-like anlagen. The epithelium of the ampullae (Fig. 371), with the exception of that in the region of the raphe, is of the squamous type. At the cristae of the ampullae, however, there is found a neuro-epithelium similar to that of the maculae. The cells adjoining both ends of the cristae are high columnar, and to these the squamous epithelium is joined. The columnar cells just mentioned form the so-called scmilunar fold. Otoliths are also present upon the neuro-epithelium of the cristae. Here the structure corresponding to the otolithic membrane of the utriculus and sacculus is called the cupula. In preserved specimens it presents the appearance of a coagulum, showing a faint striation ; in
the fresh condition, it has never been recognized as a distinct structure, at least in the lower classes of vertebrates.
Fig- 37L Part of a vertical section through the anterior ampulla, showing the membranous wall, a portion of the "crista acustica," and the "planum semilunatum" (after Retzius) : a, Semilunar fold ; 6, crista acustica ; t, nerve-fibers ; d, bloodvessels.
3. The Cochlea
The cochlea consists of an osseous portion, the bony cochlea, a membranous portion, the cochlear duct, and two perilymphatic canals. The bony cochlea consists of a central bony axis of conical shape, the modiolus, around which is wound a spiral bony canal, having in man a little over two and one-half turns, the modiolus forming the inner wall of this canal. The summit of the cochlea, which has the shape of a blunt cone, is formed by the blind end of this bony canal, and is known as the cupola. The modiolus further gives support to a spiral plate of bone, the lamina spiralis ossea, which extends from the lower part of the modiolus, and, forming two and one-half spiral turns, reaches its top, where it ends in a hook-like process, the hamulus. This bony spiral lamina partly divides the bony cochlear canal into two parts, the division being completed by a fibrous tissue membrane, the lamina spiralis membranacca, which extends from the free edge of the osseous spiral lamina to a thickened periosteal ridge, the ligamentum spirale, lining the outer wall of the bony cochlear canal. The canal above the lamina spiralis (bony and membranous) is known as the scala vestibuli, that below as the scala tympani. Both are perilymphatic canals, and communicate in the region of the last half-turn of the cochlea, by means of a narrow canal, the helicotrema, partly surrounded by the termination of the bony spiral lamina, the hamulus. The scala vestibuli is in free communication with the perilymphatic space of the vestibule ; while the scala tympani communicates with perivascular spaces surrounding the veins of the cochlear aqueduct, which latter empty into the jugular veins. The scala tympani terminates at the secondary tympanic membrane, closing the fenestra rotunda.
The cochlear duct, which, as will be remembered, communicates with the sacculus by means of the canalis reuniens, is a long tube closed at both ends,' the one end representing the vestibular sac, or ccecum vestibulare, and the other the cupolar extremity, or cacum cupolare, also known as the lagena. The cochlear duct forms about two and three-fourths spiral turns, its length being about 3.5 mm. Its diameter gradually increases from its lower to its upper or distal extremity. The cochlear duct lies above the lamina spiralis, and, in a section of the cochlea parallel to the long axis of the modiolus, it is of nearly triangular shape, with the somewhat rounded apex of the triangle attached to the osseous lamina spiralis. In the cochlear duct we may distinguish the following parts : (i) the outer wall, which is intimately connected with the periosteum of the bony cochlear canal ; (2) the tympanal wall, resting on the membranous basilar membrane, with its highly differentiated neuro-epithelium, the spiral organ of Corti ; and (3) the vestibular wall, bordering on the scala vestibuli, the intervening structures forming a very delicate membrane the vestibular or Rcissnci- ' s membrane.
From the account given thus far, it may be seen that within the bony cochlear canal there are found three membranous canals, running parallel with one another and with the osseous lamina spiralis about which they are grouped. Two of these membranous canals, the scala vestibuli and the scala tympani, are perilymphatic spaces, and are consequently lined by endothelial cells ; between them is found the cochlear duct, from its position known also as the scala media, lined by epithelial cells. These three membranous canals retain their relative position in their spiral course about the modiolus, and, in a section through the cochlea parallel to the bony axis of the modiolus, would be met with at each turn, and at each turn present essentially the same relative position and structure. In figure 372, which is from a longitudinal section of the cochlea of a cat, the general relations of the parts are clearly shown. Figure 373 is sketched from a longitudinal section of the cochlea of a guineapig, and shows the appearance presented by a section through one of the turns of the bony cochlear canal and its contents as seen under higher magnification. We may now proceed with a fuller consideration of the structures mentioned.
Fig. 372. Longitudinal section of the cochlea of a cat ; X 2 5- This figure giresa general view of the cochlea. The cochlear duct is met with six times in the section : dc t cochlear duct ; gsp, spiral ganglion ; Kn, osseous cochlear wall ; Isp, ligamentum spirale ; msp, membrana spiralis; mv, membrana vestibularis or Reissner's membrane ; , nervus cochlearis; set, scala tympani; scv, scala vestibuli (Sobotta, "Atlas and Epitome of Histology").
The lamina spiralis ossea consists of two bony plates which inclose between them the ramifications of the cochlear nerve. The vestibular surface of the osseous lamina spiralis is covered by periosteum, which is continuous with a peculiar tissue, known as limbus spiralis. The latter begins at the point of attachment of Reissner's membrane, extends peripherally (externally), and ends in two sharp ridges, of which the shorter, the labium vcstibulare, projects into the inner space of the cochlear duct and continues into the tectorial membrane ; while the other and longer, the labium tympanicum, becomes attached to the wall of the scala tympani and continues into the basilar membrane. Between the two ridges is a sulcus, the sulcus spiralis interims. (Fig- 373-) The limbus spiralis
Fig. 373. Section through one of the turns of the osseous and membranous cochlear ducts of the cochlea of a guinea-pig ; X 9 : ?> Scala vestibuli ; m, labium vestibulare of the limbus ; , sulcus spiralis internus ; o, nerve-fibers lying in the lamina spiralis ; /, ganglion cells ; q, blood-vessels ; a, bone ; b, Reissner's membrane ; DC, ductus, cochlearis ; d, Corti's membrane;/", prominentia spiralis; g, organ of Cord; h, ligamentum spirale ; i, crista basilaris ; k, scala tympani.
is a connective-tissue formation in the region of the cochlear duct connected with the periosteum of the osseous spiral lamina and extending from the point of attachment of Reissner's membrane to the labium tympanicum. The tissue of the limbus spiralis is dense and richly cellular, and simulates in its structure the substantia propria of the cornea. A casual view would seem to disclose a high columnar epithelium, but upon closer observation, it is seen that the cellular elements are interspersed with fibers which extend to the surface. Some investigators regard this tissue as fibrocartilage ; others, again, as a tissue sui generis, consisting of epithelial cells mingled with connective-tissue fibers. If the labium vestibulare of the limbus spiralis be examined from the vestibular surface, a number of irregular tubercles are seen at its inner portion (near Reissner's membrane), while at its outer portion long, radially disposed ridges may be observed, the so-called auditory teeth of Huschke. The connective-tissue wall of the sulcus spiralis internus consists of a nonnucleated fibrillar tissue which is continued into the labium tympanicum. The latter is perforated by nerves, thus giving rise at this point to the foramina nervosa,
Between the point of attachment of Reissner's membrane and the labium vestibulare, the superficial epithelium of the limbus spiralis is flat, and lines the auditory teeth and the depressions between them in a continuous layer. The epithelium of the sulcus spiralis internus is somewhat higher.
The ligamentum spirale forms the thickened periosteum of the outer wall of the osseous cochlear canal. It presents two inwardly projecting ridges, the crista basilaris, to which the membranous lamina spiralis is attached, and the promincntia spiralis, which contains one or several blood-vessels ; between the two ridges lies the sulcus spiralis externus. The portion of the ligamentum spirale forming the periosteum of the bony cochlear canal consists of a fibrous tissue containing many nuclei, but changes internally into a looser connective tissue. The connective tissue lying external to the outer wall of the cochlear duct is veiy dense and rich in cellular elements and blood-vessels, but in the crista basilaris it changes to a hyaline, noncellular tissue, continuous with the lamina basilaris. That portion of the spiral ligament lying between the prominentia spiralis and the attachment of Reissner's membrane is known as the stria vascularis. The epithelium covering this area (a portion of the epithelium lining the cochlear duct) consists of columnar, darkly granulated cells, which now and then are arranged so as to present the appearance of a stratified epithelium, but which is more correctly interpreted as an epithelium of the pseudostratified variety. This epithelium shows no distinct demarcation from the underlying connective tissue. Beneath this epithelium there is found a rich capillary network, certain loops of which extend into the epithelium (Retzius). It is thought that the stria vascularis is concerned in the formation of the endolymph of the cochlear duct.
The membranous lamina spiralis, or the basilar membrane, extends from the tympanic lip of the osseous spiral lamina to the crista basilaris of the ligamentum spirale.
As already stated, the tissue composing the labium tympanicum of the limbus extends into the basilar membrane. In this membrane the surface toward the cochlear duct is known as the cochlear surface, that toward the scala tympani as the tympanic surface. Two layers are differentiated in the basilar membrane, the lamina basilaris propria and the tympanic investing layer. The lamina propria consists, in turn, of (i) radially arranged basilar fibers, or acoustic strings ; (2) two thin strata of a homogeneous substance, one above and the other below the layer of basilar fibers, the upper of which is the thicker and nucleated ; and (3) a fine cuticula, of epithelial origin, lying on the cochlear side. The tympanic investing layer is highly developed in youth, but later becomes thinner, and may then be differentiated into a connective-tissue layer, regarded as a periosteal continuation of the tympanic portion of the osseous lamina spiralis, and an endothelial cell layer belonging to the lining of the perilymphatic space or the scala tympani. In the vicinity of the labium tympanicum is a bloodvessel situated within the tympanic investing layer of the basilar membrane the vas spirale.
Reissner's membrane consists of an exceedingly thin connective- ' tissue lamella, lined on the side of the cochlear duct by a layer of flattened epithelial cells and on the vestibular side by a layer of endothelial cells. The epithelium lining the cochlear duct is occasionally raised into small villus-like projections.
The Organ of Corti. In the region of the labium tympanicum of the limbus spiralis and in the greater portion of the adjoining basilar membrane, the epithelium of the cochlear duct is peculiarly modified, forming here a neuro-epithelium, which receives the terminal ramifications of the cochlear nerve and is known as the spiral organ of Corti.
Passing from the labium tympanicum to the ligamentum spirale, the following three regions may be recognized in the organ of Corti : An inner region, composed of the inner sustentacular cells and the inner auditory cells ; a middle region, consisting of the arches of Corti ; and an outer region, in which are found the outer auditory cells and the outer sustentacular cells or Deiters's cells. Two cuticular membranes are in close relationship to the organ of Corti : namely, the lamina reticrdaris and the mcmbrana tcctoria, or membrane of Corti.
In figure 374, a sketch of the organ of Corti and adjacent structures, it may be observed that the epithelium lining the sulcus spiralis internus (at the right of the figure) is of the pavement variety, and that the epithelium becomes gradually thicker until the organ of Corti is reached, where it becomes suddenly elevated in the form of a wall. In this, two varieties of cells are distinguished sustentacular cells and inner auditory cells. The sustentacular cells, which follow the flattened cells, become gradually higher from within outward and occupy three or four rows. Next come the inner auditory cells, cylindric elements, somewhat rounded and thickened at their nucleated basilar ends. The latter do not extend to the basilar membrane but end at about the level of the center of the inner pillars. At the free end of each cell is an elliptic cuticular zone, somewhat broader than the end-surface of the corresponding cell. In man about twenty rigid filaments, known as auditory hairs, are found resting on each elliptic cuticular zone. These are either arranged in a straight row or they describe a slight curve.
The middle division of the organ of Corti, the arches of Corti, consists of long slender structures, known as pillar cells, or, briefly, pillars, resting firmly upon the basilar membrane and forming an arch at the vestibular side of the latter. They surround, by the union of their free ends, a space which, as seen in figure 374> appears triangular in section. This is the tunnel of Corti.
Fig. 374- Organ of Corti : At x the tectorial membrane is raised ; c, outer sustentacular cells ; d, outer auditory cells ; f, outer pillar cells ; g, tectorial membrane ; //, inner sustentacular cells; i,p, epithelium of the sulcus spiralis internus ; k, labium vestibulare ; e, tympanic investing layer ; m, outer auditory cells ; , , nerve-fibers which extend through the tunnel of Corti ; o, inner pillar cell ; q, nerve-fibers ; /;, b, basilar membrane ; a, epithelium of the sulcus spiralis externus ; r, cells of Hensen ; s, inner auditory cell ; /, ligamentum spirale (after Retzius).
According to their position, w 7 e distinguish inner and outer pillars, the inner being more numerous than the outer. Including the entire extent of the lamina spiralis membranacea, we find that there are about 6000 of the inner and 4500 of the outer pillar cells.
Each pillar cell originates from an epithelial cell, and is found to be composed of a protoplasmic portion containing the nucleus, which may be regarded as a remnant of the primitive cell, and of a cuticular formation derived from the primitive cell, forming the elongated body of the pillar cell the pillar. The free adjoining ends are called the heads of the pillars. The head of the inner pillar is provided with a flattened process, the head-plate, which extends outward and forms an obtuse angle with the axis of the pillar. Under this plate, and at the outer side of the head of the inner pillar, is a depression into which fits the head of the outer pillar. The latter also extends outward in the shape of a phalangeal plate, with a thinner process, the phalangeal process, at its end. The phalangeal plate and process lie under the head-plate of the inner pillar, the process extending a little beyond this, forming an acute angle with the head of the outer pillar. At the inner side of the head of the outer pillar is a convex articular surface, with which, as a rule, two, and occasionally even three, articular surfaces of the inner pillars come in contact. The outer and inner pillars appear to possess an indistinct longitudinal striation, and their basilar plates are continuous with the extremely fine cuticula covering the basilar membrane. The inner margins of the basilar plates belonging to the inner pillars border on the foramina nervosa ; while the outer margins of the basilar plates belonging to the outer pillars come in contact with the basal end of the innermost row of the cells of Deiters in the outer region of Corti's organ. The protoplasmic portions of the pillar cells, constituting what are known as basal cells, lie against the basilar plates of the corresponding pillars, i. e., on the basilar membrane, and partly cover the bodies of the pillars, especially the surfaces toward the tunnel.
In order to comprehend the relative position of the inner auditory cells to the inner pillars, it may be stated that one auditory cell rests upon every two inner pillars.
The outer region of Corti's organ is joined directly to the outer pillar cells, and consists of four rows of auditory cells alternating with an equal number of sustentacular cells or Deiters's cells. Following these structures and in contact with them are the outermost sustentacular cells, known as Hensen's cells.
The outer auditory cells have a structure similar to that of the inner auditory cells, but possess a more slender body. They do not extend as far as the basilar membrane, but end at a distance from the latter equal to about double their own length. The cuticular zone of each outer auditory cell likewise assumes the form of an ellipse, with its long axis pointing radially. The surface of this zone also is provided with about twenty stiff auditory hairs, arranged in the form of a decidedly convex arch, the convexity of which points outward. At a short distance from the cuticular zone of each outer auditory cell is a peculiar round body, found only in these cells, the significance of which is unknown.
Deiters's cells rest on the basilar membrane, and in shape resemble a flask with a narrow neck, known as the phalangeal process, the latter lying between the auditory cells. The nuclei of Deiters's cells lie in the upper parts of the thickened basal portions of these cells.
With each Deiters's cell there is associated a cuticular structure, which extends along the surface of each cell in the form of a thin fiber, the sustentacular fiber, and which is found partly within and partly without the cell. The sustentacular fiber begins near the center of the thicker basal portion of the cell-body and extends first into the cell itself, then passes to the surface, and, entering the phalangeal process, passes to the top of the cell and expands as a plate, to which the name phalangeal plate has been given. The latter is broader than the phalangeal process, and since, as we shall see, the phalangeal plates are joined to one another, as well as to the elliptically shaped cuticular zones of the outer auditory cells, there remains a space between the cells of Deiters and the auditory cells, as also between the outer pillars and the innermost of the outer auditory cells, known as Nuel's space. To the basal regions of the inner row of the cells of Deiters is joined the basal plate of the outer pillars of the arches of Corti.
Next to the outer row of Deiters's cells are the cells of Hensen, arranged in about eight radially disposed rows. They form an eminence which is high internally, but gradually decreases in height externally. The somewhat narrowed bases of Hensen's cells probably extend, without exception, to the basilar membrane. The free surfaces of these cells are likewise covered by a thin cuticular membrane. In man the cells of Hensen usually contain yellow pigment ; in the guinea-pig, as a rule, fat ; and in the rabbit, generally rudiments of sustentacular fibers. Externally the cells of Hensen grad^ ally change into elements of a more cuboid type the cells of Claudius, of which there are about ten rows, radially disposed. The surfaces of the latter also possess a cuticular margin ; the nucleus is at the center of each cell and pigment is also present. Darker elements with more basally situated nuclei sometimes occur between these cells, giving rise to the appearance of a double-layered epithelium (Bottcher's cells).
Thus far we have considered in detail the cells comprising the organ of Corti, and described their relative positions and sequence from within outward. In order to give a clearer understanding of the mutual relations of these cells, from within outward and in the direction of the spiral turning of the cochlea, we shall now consider the appearance presented in a surface view of the organ of Corti.
From within outward a surface view of the organ of Corti presents the following characteristics : The somewhat broadened hexagonal outlines of the inner sustentacular cells adjoin the epithelial elements of the sulcus spiralis internus and terminate externally in a spiral undulating line (if seen for only a short distance, this line appears straight). On this line border the contours of the cuticular zones belonging to the inner auditory cells. The outer margins of the cuticular zones come in contact with the head-plates of the inner pillars, the cuticular zone of one inner auditory cell coming in contact with at least two head-plates. The externally directed processes of the head-plates belonging to the inner pillars come in contact with one another and end in a spiral line which for a short distance is apparently straight. The head-plates of the inner pillars cover the head-plates of the outer pillars (which also come in contact with each other), also their phalangeal plates, but not their phalangeal processes, which thus project beyond the line formed by the outer borders of the head-plates of the inner pillars. It should be mentioned that about three head-plates belonging to the inner pillar cells are in apposition to every two head-plates and their phalangeal processes of the outer pillar cells. The succeeding four rows, from within outward, are made up of alternately placed cuticular zones of the outer hair cells and the phalangeal plates of the Deiters's cells, alternating like the squares of a chess-board. This regular arrangement is lost in the outer row of Deiters's cells. The cells of Hensen adjoin this row, and when viewed from the surface, present the appearance of irregular polygons.
This arrangement is, however, seldom found to be as typical as that just described ; although the relations of the cells to one another always correspond in general to the foregoing scheme.
In the cupolar and vestibular sacs the neuro-epithelium changes into an epithelium of an indifferent type.
The lamina reticularis is formed by the cementing together of the phalangeal processes of the outer pillars and the phalangeal plates of Deiters's cells, and is continued externally by a cuticular membrane which covers the cells of Hensen and, as a much thinner cuticular membrane, extends over the cells of Claudius. In this membrane there are found three or four rows of small apertures, into which the outer hair cells project.
The membrana tectoria Cortii is attached to the limbus spiralis, but becomes free at the margin of the labium vestibulare and thickens considerably, again becoming thinner toward its free end.
Fig. 375. Surface of the organ of Corti, with the surrounding structures, from the basal turn of the cochlea of a new-born child ; the original drawing reduced one-half (after Retzius, 84): a, Epithelium of the sulcus spiralis externus ; b, Hensen' s cells; c, terminal frame; d, phalanges ; /, outer auditory cells; g, flattened processes of the outer pillar cells ; h, flattened processes of the inner pillar cells ; i, inner auditory cells ; k, inner sustentacular cells ; /, epithelium of the sulcus spiralis interims ; in, margin of the labium vestibulare ; , epithelium of the limbus laminae spiralis ; o, line of attachment of the membrana Reissneri ; /, epithelium of the membrana Reissneri, the latter inverted.
Hence an inner attached and an outer free zone may be differentiated. This membrane has no nuclei, and shows a fine radial striation. Its free portion bridges over the sulcus spiralis internus and rests upon the organ of Corti. Its outer margin extends as far as the cells of Hensen. The development of this membrane is not thoroughly understood, although it very probably represents a displaced cuticular formation belonging to the cells of the limbus spiralis. This acceptation has recently been confirmed (Exner).
The auditory nerve gives off, soon after entering the internal auditory meatus, vestibular branches to the maculae in the utriculus and sacculus and to the cristae in the semicircular canals, and a cochlear branch, which passes up through the modiolus in anastomosing bony canals. From this centrally placed column of nervefibers, a continuous sheet of nerve -fibers, arranged in the form of anastomosing bundles, passes radially into the osseous spiral lamina and thence to the organ of Corti. Near the base of the osseous spiral lamina, along the entire length of this sheet of nerve-fibers, there is situated in a special bony canal a ganglion, known as the spiral ganglion of the cochlea. The ganglion cells of this ganglion are bipolar, one of the processes of each cell, the dendrite, extending outward through the osseous spiral lamina to the organ of Corti, the other process, the neuraxis, passing through the bony canal in the modiolus, through the internal auditory meatus, and thence to the medulla. The dendritic processes of the nerve-cells of the spiral ganglion form bundles of medullated nerve-fibers, which pass outward within the osseous spiral lamina, forming, in the outer portion of the latter, a closely meshed plexus, from which small bundles of nerve-fibers proceed through the foramina nervosa of the labium tympanicum to the organ of Corti ; immediately before passing through these foramina, the medullated nerve-fibers lose their medullary sheaths and neurilemma.
These nonmedullated fibers, with or without further dividing, are then arranged in small bundles, which, for a certain distance, have a spiral course : that is to say, parallel to the tunnel of Corti. One such spiral bundle is situated on the inner side of the inner pillars, under the inner row of hair cells ; another, on the outer side of the inner pillars, in the tunnel of Corti. Other fibers pass through the tunnel of Corti, so-called tunnel-fibers, to reach the outer side of the arches of Corti, where they are arranged in three or four spiral bundles, at the outer side of the outer pillars and between the rows of the cells of Deiters. From the nerve -fibers of these spirally arranged bundles, terminal branches are given off, which terminate, after further division, on the inner and outer hair cells (Retzius, Geberg).
Regarding the blood-vessels of the membranous labyrinth, it should be mentioned that the internal auditory artery is a branch of the basilar artery, and divides into the rami vestibulares and rami cochleares. The branches of the former accompany those of the auditory nerve as far as the utriculus and sacculus. At the maculae and cristae the capillary networks are numerous and finely meshed, but in the remaining portions of the utriculus, sacculus, and semicircular canals, they form coarser networks. The cochlear branch accompanies the divisions of the auditory nerve as far as the first spiral turn of the cochlea ; the arteries supplying the remaining turns enter the axis of the modiolus, where they divide into numerous branches. The latter are coiled in a peculiar manner, forming the so-called glomeruli arteriosi cochlea. From these, branches are given off which penetrate the vestibular wall of the lamina spiralis ossea, where they supply the limbus spiralis and the small quantity of connective tissue in the membrana vestibularis. Other branches surround the scala vestibuli, supply the walls of the latter, and then continue to the ligamentum spirale, the stria vascularis, and the lamina basilaris.
Fig. 376. Scheme of distribution of blood-vessels in labyrinth (after Eichler) : g, Artery ; h, spiral ganglion ; z, vein ; v, scala vestibuli ; DC, ductus cochlearis ; c, capillaries in the ligamentum spirale ; d, capillaries in the limbus spiralis ; f, scala tympani.
The venous trunks lie close to the arteries and receive their blood from the veins which lie at the tympanal surface of the lamina spiralis and from those which encircle the outer wall of the scala tympani. The former, in turn, receive their blood from the capillaries of the limbus spiralis ; the latter, principally from the region of the ligamentum spirale and the basilar membrane.
From this description it is seen that the arterial channels are connected with the scala vestibuli, the venous with the scala tympani, and that the inner blood stream circulating through the lamina spiralis and limbus spiralis is separated from the blood current of the two scalae, the ligamentum spirale, and the crista basilaris (Eichler).
The entire membranous labyrinth is filled with endolympJi. The ductus endolymphaticus is, as will be remembered, a canal ending under the dura in a saccus endolympJiaticus. In connection with the latter are epithelial tubules bordering upon lymph-channels, with which they probably communicate by means of interepithelial (intercellular) spaces (Riidinger, 88). The efferent channels for the perilymph of the vestibule extend along the nerve sheaths of those nerves supplying the maculae and cristae ; these passageways finally communicate with the subdural or subarachnoid spaces. The perilymph of the cochlea is carried off by the adventitious tissue of the vena aqueductus cochleae, the lymph-vessels of which empty into certain subperiosteal lymph-channels near the inner margin of the jugular fossa.
4. On The Development of the Labyrinth
In man the epithelium lining the membranous labyrinth originates from the ectoderm as a single-layered epithelial vesicle, the auditory vesicle or the otocyst, during the fourth week of embryonic life. After being constricted off from the ectoderm, this vesicle lies in the vicinity of the epencephalon and is surrounded by mesenchyme. The auditory vesicle then develops a dorsomesial evagination, which gradually grows larger and finally becomes the ductus endolymphaticus. An evagination also occurs in the ventral wall of the vesicle, the recessus cochlea. At the same time the mesial wall is pushed inward, thus incompletely dividing the vesicle into two smaller sacs the dorsal utriculus and the ventral sacculus. From the utricular portion there arises a horizontal evagination, flat and quite broad the first trace of the lateral or horizontal semicircular canal ; soon after, another evagination, vertical and still broader than the first, is seen the anlage of the other two canals. The outer portion of these pouches gradually expands, while in the middle, the two layers of each evagination come in contact with each other and coalesce, finally becoming absorbed. In the vertical evagination two such areas of adherence are found, thus forming a superior and a posterior canal, both having a common crus at one end.
The recessus cochleae grows both in a longitudinal and in a spiral direction, forming the cochlear duct.
In the immediate vicinity of the membranous labyrinth, the mesenchyme is differentiated into a connective-tissue wall for the former. The successive layers of mesenchyme, except in those areas where the membranous labyrinth later becomes adherent to the osseous, are transformed into a mucous connective tissue. The latter is surrounded by a more compact tissue, from which are derived, first, cartilage ; then bone and periosteum, and thus, finally, the osseous labyrinth. By a peculiar process of regressive metamorphosis most of the mucous connective tissue later disappears. In the adult it is replaced by the perilymphatic spaces of the labyrinth.
In the treatment of the external and middle ear the usual methods are employed. For the study of the epithelium in conjunction with the adjacent bone the tissue is fixed and then decalcified, or subjected to those fixing methods which accomplish both processes at the same time. The latter method, however, can be applied only to very small objects.
The manipulation of the membranous labyrinth, especially that of the adult, is a very difficult technical problem. Its isolation from the petrous portion of the temporal bone without injury can be accomplished only in well-advanced fetuses and in children, and even here a thorough knowledge of the situation of the parts in the petrous portion of the temporal bone is essential. Smaller animals, especially rodents, afford better specimens. In the latter, the semicircular canals and cochlea give rise to more or less distinct projections into the tympanic cavity. If the latter be opened, the situation of the parts may be ascertained from without. In the rabbit and guinea-pig, the entire cochlea projects into the tympanic cavity, and may be easily removed in toto with a strong knife, and, as the bony cochlea in these animals has very thin walls, it offers very little resistance to the decalcifying fluid (use, for instance, 3% nitric acid).
According to Ranvier's method (89), the cochlea is opened with a scalpel in a 2 cc solution of osmic acid in normal salt solution. After twelve hours the cochlea is placed for decalcification in 2 % chromic acid, which is frequently changed. In guinea-pigs, for instance, decalcification is accomplished in a week.
According to the method of Retzius (84), the opened cochlea is treated for half an hour with a 0.5% aqueous solution of osmic acid, and then for the same length of time with a 0.5% aqueous solution of gold chlorid. The organ of Corti is then dissected out and examined as a whole, or cut after carefully removing the bone.
The labyrinth of the human adult is usually prepared as follows : The apex of the petrous portion of the temporal bone is removed and the upper semicircular canal, together with the cochlea, opened in Miiller's fluid ; in this solution the pyramid is left for three weeks ; during the first week the fluid is changed daily, and every two days during the following weeks. The specimen is then washed for twenty-four hours in running water, placed in 80% alcohol for two weeks, and finally in 96% alcohol for two days. The preparation is now ready for decalcification. This is done with 5% nitric acid, which is to be changed daily (ten days to two weeks). Then follows washing for two days in running water, carrying over into 80 % alcohol for twenty-four hours, then into 96% alcohol for from six to eight days, and, finally, infiltration and imbedding in celloidin (A. Scheibe).
The following method may also be employed with good results : The isolated pyramid with opened semicircular canal and cochlea is treated with Miiller's fluid for two days at room-temperature, and then for three weeks in a thermostat at 23 C. During the latter period, the fluid should be changed. The specimen is then washed for forty -eight hours in running water, treated for fourteen days with 80% alcohol, then for eight days with 96% alcohol, decalcified, and further treated as in the preceding method.
Up to the present time it has been customary to cut sections in celloidin ; but the combined celloidin-paraffin method may also be employed with good results, and even the paraffin method, if great care be exercised in imbedding the tissue.
The nerve-fibers and nerve-endings of the cochlea may be stained with the chrome-silver method. For this purpose it is recommended to employ embryos or young fetuses.
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Reference: Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.
Cite this page: Hill, M.A. (2020, January 25) Embryology Book - A textbook of histology, including microscopic technic (1910) Special Histology 9. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_A_textbook_of_histology,_including_microscopic_technic_(1910)_Special_Histology_9
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